1. Field of the Invention
The present invention relates to a solid polymer electrolyte having high-durability, and more particularly, to the solid polymer electrolyte having high-durability, which is excellent in oxidation resistance and preferably employed as a solid polymer electrolyte membrane for use in a polymer electrolyte fuel cell, a water electrolysis cell and the like.
2. Description of Related Art
Solid polymer electrolyte is the solid polymer material, in which the polymer chains have electrolyte groups, such as sulfonic acid groups, carboxylic acid groups and the like. The solid polymer electrolyte forms a strong bond with a specific ion and causes cations or anions to pass through selectively. The solid polymer electrolyte, therefore, is formed into particles, fibers, films or the like, and then is utilized for various purposes, such as electrodialysis, diffuse dialysis, and a battery.
In such backgrounds, for example, the above mentioned solid polymer electrolyte acts as a polymer electrolyte membrane for use in a polymer electrolyte fuel cell and a water electrolysis cell. In this case, the polymer electrolyte fuel cell comprises a proton-conducting solid polymer electrolyte membrane, the both surfaces of which are provided with a pair of electrodes. With such structure, the polymer electrolyte fuel cell produces electromotive force in a manner of supplying pure H2 or reformate H2 gases to one electrode (an anode) as fuel gases, and supplying O2 gases or air to another electrode as oxidant. Water electrolysis is a process for producing H2 and O2 by electrolyzing water by using a solid polymer electrolyte membrane.
On the other hand, in case of the above-mentioned polymer electrolyte fuel cell and water electrolysis cell, peroxide is produced by the electrode reaction, at a catalyst layer formed on a boundary face between a solid polymer electrolyte membrane and an electrode. Then peroxide diffuses, and then, the radical reaction occurs to form peroxide radicals, thereby causing the electrolyte to be degraded disadvantageously. Forming peroxide radicals is promoted by metal ions (Fe2+ and Cu2+ and the like) effluent from a tube for supplying mist, blended with supplied-gases for keeping electrolyte wetting.
To solve such problems, various electrolytes have been developed that are excellent in oxidation resistance. Particularly, perfluorosulfonic acid polymers known under the trade name of xe2x80x9cNafionxe2x80x9d available from E.I. du Pont de Nemours and Company, have extremely-high chemical stability and are hardly oxidized in the presence of peroxide (hydrogen peroxide H2O2), because they are the perfluorinated electrolyte materials having Cxe2x80x94F bond. Thus perfluorosulfonic acid polymers are excellent in property.
However, there is such problem that it is difficult to manufacture the perfluorinated perfluorosulfonic acid polymers, and it costs too much to obtain the raw materials because of its difficulty in mass production. Accordingly, use of the electrolyte membrane made of perfluorosulfonic acid polymer materials is limited to the special destination, such as a solid polymer fuel cell for space or military and the like, so it makes difficult to apply perfluorosulfonic acid polymer materials to unmilitary demands, such as a solid polymer fuel cell for a low-pollution drive source for automobile.
While, another materials have been examined and researched, instead of wholly fluorinated perfluorosulfonic, acid polymers. For example, following are listed up as a polymer electrolyte other than a fluorinated electrolyte: a crosslinked polystyrene-grafted resin membrane introducing sulfonic acid groups disclosed in Swiss patent application No. 02 636/93-6, a polyethersulfone resin membrane introducing sulfonic acid groups disclosed in Japanese patent publication laid-open No. Hei 10-45913, and the like. In addition, Japanese patent publication laid-open No. Hei 9-102322 discloses a sulfonic acid type ethylene-tetrafluoroethylene (ETFE) copolymer-graft-polystyrene membrane, which comprises the main chains formed by copolymerization of a fluorocarbon-based vinyl monomer and a hydrocarbon vinyl monomer and the hydrocarbon-based side chains containing sulfonic acid groups.
Furthermore, U.S. Pat. No. 4,012,303 and U.S. Pat. No. 4,605,685 disclose a sulfonic acid type ETFE-graft-poly(trifluorostyrene) membrane, which is prepared by graft polymerization of xcex1,xcex2,xcex2-trifluorostyrene and the membrane prepared by copolymerization of a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer, then introducing sulfonic acid groups into the resulting membrane to prepare the desired solid polymer electrolyte membrane. In this process, xcex1,xcex2,xcex2-trifluorostyrene, produced by styrene fluorination, is employed instead of styrene on the assumption that chemical stability of the side chains introducing sulfonic acid groups in polystyrene is insufficient.
These electrolyte materials, however, such as a non-fluorinated electrolyte membrane, for example, the crosslinked polystyrene-grafted resin membrane introducing sulfonic acid groups disclosed in Swiss patent application No. 02 636/93-6, the polyether sulfone resin membrane introducing sulfonic acid groups disclosed in Japanese patent publication laid-open No. Hei 10-45913 and the like, can advantageously be manufactured easier at lower cost than the wholly fluorinated electrolyte membrane represented by Nafion, but the non-fluorinated electrolyte membrane is easily degraded by peroxide produced by the electrode reaction, thus the oxidation resistance of which has been controlled to be low disadvantageously. Because the non-fluorinated compounds have the hydrocarbon structure susceptible to the oxidation reaction caused by peroxide radicals.
Furthermore, the sulfonic acid type ETFE-graft-polystyrene membrane disclosed in Japanese patent publication Laid-open No. Hei 9-102322 can be obtained at a low price and robust enough to function as a solid polymer electrolyte membrane for use in a fuel cell, in addition to this, a conductivity of which can be improved by increasing an introducing amount of sulfonic acid groups. Furthermore, an oxidation resistance of the main chains produced by copolymerization between a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer is sufficiently high, but the side chains introducing sulfonic acid groups is a hydrocarbon-based polymer susceptible to oxidation and degradation. Accordingly, application of the sulfonic acid type ETFE-graft-polystyrene membrane to a fuel cell causes the oxidation resistance of whole membrane to be insufficient, thus resulting in poor durability disadvantageously.
Furthermore, in case of utilizing a sulfonic acid type ETFE-graft-poly (trifluorostyrene) membrane disclosed in U.S. Pat. No. 4,012,303 and the like, it is considered that the above-mentioned problem is solved because the side chains thereof are composed of fluorine-based polymers. However, the base material of the side chains, xcex1,xcex2,xcex2-trifluorostyrene, is difficult to synthesize, thus it will be costly to apply the same to a solid polymer electrolyte membrane for use in a fuel cell, as similar to the above-mentioned Nafion. In addition, xcex1,xcex2, xcex2-trifluorostyrene is susceptible to degradation, thus it is difficult to deal with xcex1,xcex2,xcex2-trifluorostyrene, and it is not excellent in polymerization reactivity. Accordingly, an amount of xcex1,xcex2,xcex2-trifluorostyrene, which should be introduced as the graft side chains, is small, as a result, the resulting membrane has a low conductivity.
Furthermore, durability of the crosslinked polystyrene-grafted resin membrane introducing sulfonic acid groups disclosed in above-mentioned Swiss patent application is higher than that one disclosed in the above-mentioned US Patent. However, the above-mentioned problem cannot solved essentially by the above-mentioned technique in the point of improvement of polymeric oxidation resistance, because the technique prevents reduction of components, produced by degradation, by means of increasing chemical bonds.
On the other hand, for example, the Japanese patent publication Laid-open No. Hei 6-103992 discloses such technique that holds catalyst metals in an electrolyte and then decomposes peroxide for the purpose of preventing a hydrocarbon-based ion-exchange membrane from being degraded by radicals of peroxide, such as hydrogen peroxide and the like. However, the catalyst metals disclosed therein are utilized for reacting hydrogen with oxygen directly, for which platinum is usually employed. It will be very costly. Additionally, these catalyst metals basically act so as to decompose hydrogen peroxide which causes electrolyte to be degraded, while it also act as catalyst so as to produce hydrogen peroxide by the direct reaction between oxygen and hydrogen in a state that oxygen coexists with hydrogen. Accordingly, such problem arises that the catalyst metals do not effectively inhibit degradation of electrolyte.
Furthermore, for example, J. Membrane Science, 56 (1991) 143 discloses such attempt that employs a methylstyrene-based electrolyte instead of a polystyrene-based electrolyte, but its effect was limitative. DOE Report FSEC-CR-857-95 discloses examination concerning a hydrocarbon-based electrolyte membrane prepared by sulfonating the main chains which are components of aromatic polymers. The attempt was conducted on the assumption that the main chains had more excellent oxidation resistance than that of polymers having the main chains of single-chain type, but it was not effective enough. Furthermore, the Japanese patent publication Laid-open No. Hei 7-50170 discloses the technique concerning a polymer electrolyte having the main chains of polyolefin, but its durability was low.
Above-mentioned conventional arts were based on such ideas as following: one was, to make a polymer electrolyte structure difficult to be attacked from a point of the stearic hindrance; and another was to cause polymer to protect against attack by way of increasing chemical bonds. However, an oxidizing force produced by oxidizing radicals, such as hydrogen peroxide, was extremely strong, thus the conventional arts could not inhibit effectively the electrolyte degradation.
The inventors has been examined into details repeatedly for the purpose of preventing the peroxide radicals from being produced. As a result, the inventors found that generation of the peroxide radicals could be stopped by processes of trapping metal ions (mainly entering from a tube for fuel supply as described above), such as Fe2+, Cu2+ or the like, which generated radicals of peroxide such as hydrogen peroxide (H2O2) generated by a cell reaction; and then inactivating the metal ions. So as to realize the processes, the inventors thought that it would be effective to introduce the chelate groups into an electrolyte membrane by using some means.
Then the inventors examined and thus found the related techniques, for example, as following: Japanese patent publication Kokoku No. Hei B-30276 disclosing a technique utilized for rising an ion conductivity of electrolyte by processes of introducing metal cations, transition metal complex cations, quaternary ammonium cations or the like, into the solid electrolyte materials, and then causing these cations to trap and contain superoxide (O2xe2x88x92) having an ion conductivity higher than the cations; and Japanese patent publication Laid-open No. Hei 10-510090 disclosing a technique utilized for rising an ion conductivity by processes of causing the electrolyte materials to contain ion complexes composed of aromatic anion groups, and then causing the anion groups of the ion complexes to trap and eliminate cations (H+ ion) to produce anions, thereby rising the ion conductivity. These techniques, however, utilizes ion-exchange so as to trap anions or cations.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a solid polymer electrolyte having high-durability, which is excellent in oxidation resistance and, in case of utilized as a solid polymer electrolyte for use in a polymer electrolyte fuel cell and a water electrolysis device, capable of inhibiting peroxide radicals from being produced, by processes of trapping and inactivating such metal ions, in a chelate fashion, that cause radicals of peroxide, such as hydrogen peroxide (H2O2) produced by an electrode reaction.
Further, another object of the present invention is to provide a solid polymer electrolyte having high-durability, which stands long use persistently. Further, another object of the present invention is to extend the use of comparatively inexpensive materials, such as hydrocarbon-based electrolyte materials, other than Nafion.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, a solid polymer electrolyte having high-durability of the present invention comprises chelate groups that are introduced into polymer electrolyte materials in which electrolyte groups are introduced into the polymers having hydrocarbon parts. The chelate groups trap metal ions in a chelate fashion, the metal ions generating radicals of peroxide generated by an electrode reaction.
In this case, the electrolyte materials contained in the solid polymer electrolyte materials are functional groups which have electrolyte ions, such as sulfonic acid groups, carboxylic acid groups, phosphonic acid groups, sulfone imide groups and the like. Furthermore, above-mentioned electrolyte groups may preferably be strong acid groups, and more preferably be sulfonic acid groups. At a given introducing ratio, the above-listed electrolytes are introduced into the parts which are capable of introducing electrolyte groups and are components of the hydrocarbon part-containing polymers. The introducing ratio may satisfactorily be adjusted in accordance with a use, the state of use, the kinds of electrolyte groups. Preferably, the introducing ratio by equivalent weight may be within a range of 1 50 to 5000 g/eq, more preferably be within a range of 200 to 2000 g/eq. Because if the introducing ratio is less than or equal to 150 g/eq, then the swelling due to water and solvent becomes too large and/or the strength is extremely lowered. As a result, the solid polymer electrolyte materials are not good for use. If the introducing ratio is more than or equal to 5000 g/eq, then a proton-conductivity is extremely lowered with the increase in the ohmic loss. As a result, the solid polymer electrolyte materials may not function as the electrolyte materials. Preferably, the proton-conductivity may satisfactorily be more than or equal to 1xc3x9710xe2x88x922 S/cm, and more preferably, more than or equal to 5xc3x9710xe2x88x922 S/cm.
As the hydrocarbon part-containing polymer compounds, following examples are listed up: polysulfone resins, polyether sulfone resins, polyether ether ketone resins, polycarbonate resins, polyester carbonate resins, polyarylate resins, polyoxybenzoyl resins, polybenzimidazole resins, polyester ketone resins, linear phenol-formaldehyde resins, crosslinked phenol-formaldehyde resins, urea-formaldehyde resins, melamine-formaldehyde resins, linear polystyrene resins, crosslinked polystyrene resins, linear poly(trifluorostyrene) resins, crosslinked poly(trifluorostyrene) resins, poly(2,3-diphenyl-1,4-phenyleneoxide) resins, polyphenylene oxide resins, poly(allyl ether ketone)resins, poly(allylene ether sulfone) resins, poly(phenyl quinolinic acid) resins, poly(benzyl silane) resins, ethylene-tetrafluoroethylene copolymer-graft-polystyrene resins, poly(vinylidene fluoride) graft-polystyrene resins, polytetrafluoroethylene-graft-polystyrene resins, polyimide resins, polyamide resins, polyether imide resins, polyamide imide resins, polyester resins, polyurethane resins, polysiloxane resins, polysulfide resins, polyacetal resins, poly p-phenylene derivative resins, polyphenylene sulfide resins, and the like. Other than above-listed resins, preferably, the wholly aromatic resins having such main chains as to include the so-called aromatic ring may satisfactorily be copolymers formed by bonding one or more compounds selected from a group consisted of phenylene, biphenylene, and naphthalene with one or more functional groups selected from a group consisted of xe2x80x94SO2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Sxe2x80x94Sxe2x80x94, xe2x80x94C(O)xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, imide, amide, sulfonamide, ester, sulfone ester, urethane, urea and the like. Additionally, the middle of the main chains may satisfactorily contain alkyl groups, alkylene groups and the like; or may satisfactorily have polyphosphazene derivative; or may certainly block copolymers, graft copolymers, star-burst dendrimers, polymer blend, that have various polymer segments.
Particularly, the polymers prepared by graft-polymerizing styrene into the polymer partially-containing fluorine as according to claim 24, or the polymers partially-containing aromatic compounds as according to claim 25 may be obtained at a low price and have sufficient strength even if shaped into a thin-film. Furthermore, a conductivity of these polymers can be controlled easily by adjusting the electrolyte group type and the introducing amount. Accordingly, these polymers are particularly preferable as the hydrocarbon part-containing polymers. Following polymers are listed up as the polymers prepared by graft-polymerizing styrene into the polymer partially-containing fluorine: graft copolymers of ethylene-tetrafluoroethylene resins, represented by ethylene-tetrafluoroethylene copolymer-graft-polystyrene resins, that have the main chains composed of ethylene-tetrafluoroethylene copolymer resins and the side chains composed of polystyrene capable of introducing electrolyte groups. Additionally, following polymers are listed up as the polymers partially-containing aromatic compounds: polyether sulfone resins and polyether ether ketone resins.
The chelate groups, which are introduced into the polymer electrolyte, may preferably be such as to contain the phosphonic acid groups or nitrogen. In this case, it may be more preferable if the chelate groups containing nitrogen contain phosphonic acid groups or carboxylic acid groups.
The chelate groups containing phosphonic acid groups form chelate in which two oxygen atoms in phosphonic acid groups coordinate, therefore the oxidation resistance may be improved. Furthermore, the chelate groups containing nitrogen are excellent in the oxidation resistance because nitrogen has a lone-pair forming the strong ligand. In addition, the chelate groups containing both of nitrogen and phosphonic acid groups or both of nitrogen and carboxylic acid groups have the significant chelate effect because acidic groups and nitrogen atoms bring the synergistic chelate effect. Particularly, the chelate groups containing both of nitrogen and phosphonic acid groups, are more preferable because they do not lose chelate effect even under the acidic atmosphere.
As the chelate groups containing phosphonic acid groups, following may be preferable: alkylamino monophosphonic acid groups, alkylamino diphosphonic acid groups, dialkylamino monophosphonic acid groups, alkylalkylene diamine triphosphonic acid groups, and alkylimino phosphonic acid groups.
As the chelate groups containing carboxylic acid groups, following may be preferable: alkylamino monocarboxylic acid groups, alkylamino dicarboxylic acid groups, dialkylamino monocarboxylic acid groups, alkylalkylene diamine tricarboxylic acid groups and alkylimino carboxylic acid groups.
The introducing ratio of chelate groups may satisfactorily be adjusted in accordance with a use, the state of use, and the like. In this case, the proportion of chelate groups may preferably be within a range of 0.001 to 1 by mole fraction, more preferably be within a range of 0.01 to 0.8 by mole fraction, the most preferably be within a range of 0.03 to 0.5 by mole fraction. Where the value of mole fraction is found by an expression of:
      (          Mole      ⁢              xe2x80x83            ⁢      fraction        )    =            (              Mole        ⁢                  xe2x80x83                ⁢        number        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        chelate        ⁢                  xe2x80x83                ⁢        groups            )                                                          (                              Mole                ⁢                                  xe2x80x83                                ⁢                number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                electrolyte                ⁢                                  xe2x80x83                                ⁢                groups                            )                        +                                                            (                          Mole              ⁢                              xe2x80x83                            ⁢              number              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              chelate              ⁢                              xe2x80x83                            ⁢              groups                        )                              
If a mole fraction of chelate groups is less than 0.001, then trapping metal ions maybe insufficient in some cases, therefore, durability is not improved sufficiently. Furthermore, if a mole fraction of chelate groups is too high, then there is tendency for a proton conductivity to fall, disadvantageously.
Above-mentioned chelate groups may preferably be introduced into the hydrocarbon part of polymer electrolyte materials by way of either the direct chemical bonds or, mixing the polymer electrolyte materials with such compounds as to have chelate groups.
As mentioned above, introducing the chelate groups into the polymer electrolyte materials allows the polymer electrolyte materials to avoid degradation due to peroxide radicals and, to maintain a chemical stable state. Because even if the electrode reaction produces peroxide, such as hydrogen peroxide (H2O2), in a polymer electrolyte fuel cell or the like, then metal ions which, entering from a fuel-supply tube and causing peroxide to form radicals, are trapped by the chelate groups contained in the electrolyte materials, thus the metal ions become to have no relation with the radical reaction of peroxide.
The solid polymer electrolyte having high-durability of the present invention may be used in various shape, such as particles, fibers, or membrane. Among them, membrane is the most preferable for use in the electrochemical device, such as a fuel cell or a water electrolysis device. A thickness of membrane may be varied in accordance with requirements, but, usually, for use in a fuel cell, may preferably be within a range of 1 to 500 xcexcm, more preferably be within a range of 10 to 200 xcexcm, the most preferably be within a range of 20 to 100 xcexcm. The reasons are following: if a thickness of membrane is less than 1 xcexcm then strength is not enough to ensure durability; and if a thickness of membrane is more than 500 xcexcm then an ionic resistance rises up too high.
The polymer electrolyte having high-durability of the present invention can prohibit hydrogen peroxide from forming radicals and be controlled so that electrolyte may not be degraded, even if the metal ions that cause hydrogen peroxide to form radicals are mixed with the polymer electrolyte because the chelate groups trap the metal ions. Accordingly, the comparatively inexpensive materials, such as the polystyrene-based materials, the polyether-based materials, other non-fluorine based electrolyte membranes, various hydrocarbon-based electrolyte membranes, can be used even under the condition where hydrogen peroxide may be formed easily. The present invention, therefore, also brings great economical effect.